Pavement Cross Slope

The operating characteristics of vehicles on crowned pavements
are such that on cross slopes up to 2 percent, the effect on steering
is barely perceptible. A reasonably steep lateral slope is desirable to
minimize water ponding on flat sections of uncurbed pavements due
to imperfections or unequal settlement. With curbed pavements,
a steep cross slope is desirable to contain the flow of water adjacent
to the curb. The recommended pavement cross slope for usual conditions
is 2 percent. In areas of high rainfall, steeper cross slopes may
be used (see AASHTO’s A Policy on Geometric Design
of Highways and Streets).

On multilane divided highways, pavements with three or more
lanes inclined in the same direction desirably should have greater
slope across the outside lane(s) than across the two interior lanes.
The increase in slope in the outer lane(s) should be at least 0.5
percent greater than the inside lanes (i.e., slope of 2.5 percent).
In these cases, the inside lanes may be sloped flatter than normal,
typically at 1.5 percent but not less than 1.0 percent.

For tangent sections on divided highways, each pavement should
have a uniform cross slope with the high point at the edge nearest
the median. Although a uniform cross slope is preferable, on rural
sections with a wide median, the high point of the crown is sometimes
placed at the centerline of the pavement with cross slopes from
1.5 to 2 percent. At intersections, interchange ramps or in unusual
situations, the high point of the crown position may vary depending
upon drainage or other controls.

For two lane roadways, cross slope should also be adequate
to provide proper drainage. The cross slope for two lane roadways
for usual conditions is 2 percent and should not be less than 1.0 percent.

Shoulders should be sloped sufficiently to drain surface water
but not to the extent that safety concerns are created for vehicular
use. The algebraic difference of cross slope between the traveled way
and shoulder grades should not exceed 6 to 7 percent. Maximum shoulder
slope should not exceed 10 percent. Following are recommended cross
slopes for various types of shoulders:

Bituminous and concrete-surface
shoulders should be sloped from 2 to 6 percent (often the slope
rate is identical to that used on the travel lanes).

Gravel or crushed rock shoulders should be sloped from
4 to 6 percent.

Turf shoulders should be sloped at about 8 percent.

Pavement cross slopes on all roadways, exclusive of superelevation
transition sections, should not be less than 1 percent.

Median Design

A median (i.e., the area between opposing travel lane edges)
is provided primarily to separate opposing traffic streams. The
general range of median width is from 4 ft to 76 ft [1.2 m to 22.8
m], with design width dependent on the type and location of the
highway or street facility.

In rural areas, median sections are normally wider than in
urban areas. For multi-lane rural highways without access control,
a median width of 76 ft [22.8 m] is desirable to provide complete shelter
for trucks at median openings (crossovers). These wide, depressed
medians are also effective in reducing headlight glare and providing
a horizontal clearance for run-off-the-road vehicle encroachments.

Where economically feasible, freeways in rural areas should
also desirably include a 76 ft [22.8 m] median. Since freeways
by design do not allow at-grade crossings, median widths need not
be sufficient to shelter crossing trucks. In this regard, where
right-of-way costs are prohibitive, reduced median widths (less
than 76 ft [22.8 m]) may be appropriate for certain rural freeways.
Statistical studies have shown that over 90 percent of median encroachments
involve lateral distances traveled of 48 ft [14.4 m] or less. In
this regard, depressed medians on rural freeways sections should
be 48 ft [14.4 m] or more in width.

Urban freeways generally include narrower, flush medians with
continuous longitudinal barriers. For urban freeways with flush
median and six or more travel lanes, full (10 ft [3.0 m]) inside
shoulders should be provided to provide space for emergency parking.
Median widths vary up to 30 ft [9.0 m], with 24 ft [7.2 m] commonly
used. For projects involving the rehabilitation and expansion of
existing urban freeways, the provision of wide inside shoulders
may not be feasible.

For low-speed urban arterial streets, flush or curbed medians
are used. A width of 16 ft [4.8 m] will effectively accommodate
left-turning traffic for either raised or flush medians. Where
the need for dual left turns are anticipated at cross streets, the
median width should be 28 ft [8.4 m]. The two-way (continuous)
left-turn lane design is appropriate where there exists (or is expected
to exist) a high frequency of mid-block left turns. Median types
for urban arterials without access control are further discussed
in Chapter 3, Section 2, “
Urban Streets”.

When flush median designs are selected, it should be expected
that some crossing and turning movements can occur in and around
these medians. Full pavement structure designs will usually be carried
across flush medians to allow for traffic movements.

Lane Widths

For high-speed facilities such as all freeways and most rural
arterials, lane widths should be 12 ft [3.6 m] minimum. For low-speed
urban streets, 11 ft or 12 ft [3.3 m or 3.6 m] lanes are generally used.
Subsequent sections of this manual identify appropriate lane widths
for the various classes of highway and street facilities.

Bicycle accomodations should be considered when a project
is scoped. Bicycle consideration is required on urban facilities.
To accommodate bicycles, the outside curb lane should be 14 ft
[4.2m] from the lane stripe to the gutter joint or gutter lip on
a monolithic curb. For a striped bicycle lane, the clear width
is 5 ft [1.5m] minimum. For additional guidance, refer to the AASHTO Guide
for the Development of Bicycle Facilities.

Shoulder Widths

Wide, surfaced shoulders provide a suitable, all-weather area
for stopped vehicles to be clear of the travel lanes. Shoulders
are of considerable value on high-speed facilities such as freeways
and rural highways. Shoulders, in addition to serving as emergency
parking areas, lend lateral support to travel lane pavement structure,
provide a maneuvering area, increase sight distance of horizontal curves,
and give drivers a sense of safe, open roadway. Design shoulder
widths for the various classes of highways are shown in the appropriate
subsequent portions of this manual.

Shoulder widths should accomodate bicycle facilities and provide
a 1 ft offset to barriers across bridges being replaced or rehabilitated.

On urban collector and local streets, parking lanes may be
provided instead of shoulders. On arterial streets, parking lanes
decrease capacity and generally are discouraged.

Sidewalks and Pedestrian Elements

Walking is an important transportation mode that needs to
be incorporated in transportation projects. Planning for pedestrian
facilities should occur early and continuously throughout project development. Sidewalks
provide distinct separation of pedestrians and vehicles, serving
to increase pedestrian safety as well as to enhance vehicular capacity. When
any of the following factors are present, sidewalks should be included
on a project located in an urban setting where:

Construction is within existing right-of-way, and
the scope of work involves pavement widening;

Full reconstruction or new construction that requires
new right-of-way.

In typical suburban development, there are initially few pedestrian
trips because there are few closely located pedestrian destinations.
However, when pedestrian demand increases with additional development,
it may be more difficult and more costly to go back and install
pedestrian facilities if they were not considered in the initial
design. Early consideration of pedestrian facility design during
the project development process may also greatly simplify compliance
with accessibility requirements established by the Americans
with Disabilities Act Public Accessibility Guidelines for Pedestrian
Facilities in the Public Right of Way (PROWAG) and
the Texas Accessibility Standards (TAS).

Sidewalk Location. For pedestrian comfort,
especially adjacent to high speed traffic, it is desirable to provide
a buffer space between the traveled way and the sidewalk as shown
in Figure 2-7(A). For curb and gutter sections, a buffer space
of 4 ft to 6 ft [1.2m to 1.8m] between the back of the curb and
the sidewalk is desirable. Roadways in urban and suburban areas
without curb and gutter require sidewalks , which should be placed
between the ditch and the right of way line if practical. Note that
pedestrian street crossings must be ADA compliant. For roadways
functionally classified as rural, the shoulder may be used to accommodate
pedestrian and bicycle traffic. Where a shoulder serves as part
of the pedestrian access route, it must meet ADA/TAS requirements.

Sidewalk Width. Sidewalks should be wide
enough to accommodate the volume and type of pedestrian traffic
expected in the area. The minimum clear sidewalk width is 5 ft
[1525 mm]. Where a sidewalk is placed immediately adjacent to the
curb as shown in Figure 2-7(B), a sidewalk width of 6 ft [1830 mm]
is recommended to allow additional space for street and highway
hardware and allow for the proximity of moving traffic. Sidewalk
widths of 8 ft [2440 mm] or more may be appropriate in commercial
areas, along school routes, and other areas with concentrated pedestrian traffic.

Where necessary to cross a driveway while maintaining the
maximum 2 percent cross slope, the sidewalk width may be reduced
to 4 ft [1220 mm] (Figure 2-8). Also,if insufficient space is available
to locate street fixtures (elements such as sign supports, signal
poles, fire hydrants, manhole covers, and controller cabinets that
are not intended for public use) outside the 5 ft [1525 mm] minimum
clear width, the sidewalk width may be reduced to 4 ft [1220 mm] for
short distances.

Street Crossings. Intersections can
present formidable barriers to pedestrian travel. Intersection designs
which incorporate properly placed curb ramps, sidewalks, crosswalks,
pedestrian signal heads and pedestrian refuge islands can make the
environment more accommodating for pedestrians. Desirably, drainage
inlets should be located on the upstream side of crosswalks and
sidewalk ramps.

Refuge islands enhance pedestrian comfort by reducing effective
walking distances and pedestrian exposure to traffic. Islands should
be a minimum of 6 ft [1.8m] wide to afford refuge to people in wheelchairs.
A minimum 5 ft [1.5m] wide by 6 ft [1.8m] long curb ramp should
be cut through the island for pedestrian passage. Install curb ramps
with a minimum 5 ft x 5 ft [1525 mm x 1525 mm] landing in the island if
room allows, see Figure 2-9. Curb ramps and crosswalks must be aligned behind
the nose of the median island to provide adequate refuge.

Curb Ramps and Landings. Curb ramps
must be provided in conjunction with each project where the following
types of work will be performed:

reconstruction, rehabilitation
and resurfacing projects, including overlays, where a barrier exists
to a sidewalk or a prepared surface for pedestrian use

construction of curbs, curb and gutter, and/or sidewalks

installation of traffic signals which include pedestrian
signals

installation of pavement markings for pedestrian crosswalks

A sidewalk curb ramp and level landing will be provided wherever
a public sidewalk crosses a curb or other change in level. The
maximum grade for curb ramps is 8.3 percent. The maximum cross slope
for curb ramps is 2 percent. Flatter grades and slopes should be
used where possible and to allow for construction tolerances and
to improve accessibility. The preferred width of curb ramps is 5
ft [1.5m] and the minimum width is 4 ft [1.2m], exclusive of flared
sides. Where a side of a curb ramp is contiguous with a public
sidewalk or walking surface, it will be flared with a slope of 10 percent
maximum, measured parallel to the curb.

Where a perpendicular or directional curb ramp is provided,
a landing must be provided at the top of the ramp run. The slope
of the landing will not exceed 2 percent in any direction. The
landing should have a minimum clear dimension of 5 ft x 5 ft [1.5m
x 1.5m] square or accomodate a 5 ft [1.5m] diameter circle and will
connect to the continuous passage in each direction of travel as shown
in Figure 2-7. Landings may overlap with other landings.

Where a parallel curb ramp is provided (i.e., the sidewalk
ramps down to a landing at street level) a minimum 5 ft x 5 ft [1.5m
x 1.5m] landing should be provided at the entrance to the street.

The bottom of a curb ramp run should be wholly contained within
the markings of the crosswalk. There should be a minimum 4 ft x
4 ft [1.2m x 1.2m] maneuvering space wholly contained within the
crosswalk, whether marked or unmarked and outside the path of parallel
vehicular traffic.

Manhole covers, grates, and obstructions should not be located
within the curb ramp, maneuvering area, or landing.

The standard sheet PED may be referenced for additional information
on the configuration of curb ramps.

Cross Slope. Sidewalk cross slope will
not exceed 1:50 (2 percent). Due to construction tolerances, it
is recommended that sidewalk cross slopes be shown in the plans
at 1.5 percent to avoid exceeding the 2 percent limit when complete.
Cross slope requirements also apply to the continuation of the
pedestrian route through the cross walk. Sidewalks immediately
adjacent to the curb or roadway may be offset to avoid a non-conforming
cross slope at driveway aprons by diverting the sidewalk around
the apron as shown in Figure 2-8. Where the ramp sidewalk must be
sloped to cross a driveway, the designer is encouraged to use a
running slope of 5 percent or less on the sloping portions of the
sidewalk to avoid the need for handrails.

Street Furniture. Special consideration
should be given to the location of street furniture (items intended
for use by the public such as benches, public telephones, bike racks,
and parking meters). A clear ground space at least 2.5 ft x 4 ft
[760 mm x 1.2m] with a maximum slope of 2 percent must be provided
and positioned to allow for either forward or parallel approach
to the element in compliance with PROWAG/TAS. The clear ground
space must have an accessible connection to the sidewalk and must
not encroach into the 5 ft [1.5m] minimum sidewalk width by more
than 2 ft [610 mm]. Pedestrian push buttons must also be within
specified reach ranges of a ground space.

PROWAG/TAS. Specific design requirements
to accommodate the needs of persons with disabilities are established
by the PROWAG/TAS and related rulemaking. A request for a design
variance for any deviations from TAS requirements must be submitted
to the Texas Department of Licensing and Regulation (TDLR) for approval.

Curb and Curb and Gutters

Curb designs are classified as vertical or sloping. Vertical
curbs are defined as those having a vertical or nearly vertical
traffic face 6 inches [150 mm] or higher. Vertical curbs are intended
to discourage motorists from deliberately leaving the roadway.
Sloping curbs are defined as those having a sloping traffic face
6 inches [150 mm] or less in height. Sloping curbs can be readily
traversed by a motorist when necessary. A preferable height for
sloping curbs at some locations may be 4 inches [100 mm] or less
because higher curbs may drag the underside of some vehicles.

Curbs are used primarily on frontage roads, crossroads, and
low-speed streets in urban areas. They should not be used in connection
with the through, high-speed traffic lanes or ramp areas except
at the outer edge of the shoulder where needed for drainage, in
which case they should be of the sloping type.

Roadside Design

Of particular concern to the design engineer is the number
of single-vehicle, run-off-the-road accidents which occur even on
the safest facilities. About one-third of all highway fatalities
are associated with accidents of this nature. The configuration
and condition of the roadside greatly affect the extent of damages
and injuries for these accidents.

Increased safety may be realized through application of the
following principles, particularly on high-speed facilities:

A “forgiving” roadside should
be provided, free of unyielding obstacles including landscaping, drainage
facilities that create obstacles, steep slopes, utility poles, etc.
For adequate safety, it is desirable to provide an unencumbered
roadside recovery area that is as wide as practicable for the specific
highway and traffic conditions.

For existing highways, treatment of obstacles should be
considered in the following order:

Eliminate
the obstacle.

Redesign the obstacle so that it can by safely traversed.

Relocate the obstacle to a point where it is less likely
to be struck.

Make the obstacle breakaway.

Apply a cost-effective device to provide for redirection
(longitudinal barrier) or severity reduction (impact attenuators).
Barrier should only be used if the barrier is less of an obstacle
than the obstacle it would protect, or if the cost of otherwise
safety treating the obstacle is prohibitive.

Delineate the obstacle.

Use of higher than minimum design standards result in
a driver environment which is fundamentally safer because it is
more likely to compensate for driver errors. Frequently, a design, including
sight distances greater than minimum, flattened slopes, etc., costs
little more over the life of a project and increases safety and
usefulness substantially.

For improved safety performance, highway geometry and
traffic control devices should merely confirm drivers' expectations.
Unexpected situations, such as left side ramps on freeways, sharp
horizontal curvature introduced within a series of flat curves,
etc., have demonstrated adverse effects on traffic operations.

These principles have been incorporated as appropriate into
the design guidelines included herein. These principles should
be examined for their applicability at an individual site based
on its particular circumstances, including the aspects of social
impact, environmental impact, economy, and safety.

Slopes and Ditches

Sideslopes. Sideslopes refer to the
slopes of areas adjacent to the shoulder and located between the shoulder
and the right-of-way line. For safety reasons, it is desirable
to design relatively flat areas adjacent to the travelway so that
out-of-control vehicles are less likely to turn over, vault, or
impact the side of a drainage channel.

Slope Rates. The path that an out-of-control
vehicle follows after it leaves the traveled portion of the roadway
is related to a number of factors such as driver capabilities, slope
rates, and vehicular speed. Accident data indicates that approximately
75 percent of reported encroachments do not exceed a lateral distance
of 30 ft [9 m] from the travel lane edge where roadside slopes are
1V:6H or flatter - slope rates that afford drivers significant
opportunity for recovery. Crash test data further indicates that
steeper slopes (up to 1V:3H) are negotiable by drivers; however,
recovery of vehicular control on these steeper slopes is less likely.
Recommended clear zone width associated with these slopes are further
discussed in Clear Zone.

Design Values. Particularly difficult
terrain or restricted right-of-way width may require deviation from
these general guide values. Where conditions are favorable, it
is desirable to use flatter slopes to enhance roadside safety.

Front Slope.
The slope adjacent to the shoulder is called the front slope. Ideally,
the front slope should be 1V:6H or flatter, although steeper slopes
are acceptable in some locations. Rates of 1V:4H (or flatter) facilitate
efficient operation of construction and maintenance equipment.
Slope rates of 1V:3H may be used in constrained conditions. Slope
rates of 1V:2H are normally only used on bridge header banks or
ditch side slopes, both of which would likely require rip-rap.

When
the front slope is steeper than 1V:3H, a longitudinal barrier may
be considered to keep vehicles from traversing the slope. A longitudinal
barrier should not be used solely for slope protection for rates
of 1V:3H or flatter since the barrier may be more of an obstacle
than the slope. Also, since recovery is less likely on 1V:3H and
1V:4H slopes, fixed objects should not be present in the vicinity
of the toe of these slopes. Particular care should be taken in
the treatment of man-made appurtenances such as culvert ends.

Back Slope. The back slope is typically
at a slope of 1V:4H or flatter for mowing purposes. Generally,
if steep front slopes are provided, the back slopes are relatively
flat. Conversely, if flat front slopes are provided, the back slopes
may be steeper. The slope ratio of the back slope may vary depending
upon the geologic formation encountered. For example, where the
roadway alignment traverses through a rock formation area, back
slopes are typically much steeper and may be close to vertical.
Steep back slope designs should be examined for slope stability.

Design. The intersections of slope planes
in the highway cross section should be well rounded for added safety,
increased stability, and improved aesthetics. Front slopes, back
slopes, and ditches should be sodded and/or seeded where feasible
to promote stability and reduce erosion. In arid regions, concrete
or rock retards may be necessary to prevent ditch erosion.

Where guardrail is placed on side slopes, the area between
the roadway and barrier should be sloped at 1V:10H or flatter.

Roadside drainage ditches should be of sufficient width and
depth to handle the design run-off and should be at least 6 inches
[150 mm] below the subgrade crown to insure stability of the base course.
For additional information, see Drainage Facility Placement.

Lateral Offset to Obstructions

It is generally desirable that there be uniform clearance
between traffic and roadside features such as bridge railings, parapets,
retaining walls, and roadside barriers. In an urban environment,
right of way is often limited and is characterized by sidewalks,
enclosed drainage, numerous fixed objects (e.g., signs, utility
poles, luminaire supports, fire hydrants, sidewalk furniture, etc.),
and traffic making frequent stops. Uniform alignment enhances highway
safety by providing the driver with a certain level of expectation,
thus reducing driver concern for and reaction to those objects.
The distance from the edge of the traveled way, beyond which a roadside
object will not be perceived as an obstacle and result in a motorist’s
reducing speed or changing vehicle position on the roadway, is called
the lateral offset. This lateral offset to obstructions helps to:

Avoid impacts on vehicle lane position and encroachments
into opposing or adjacent lanes

As a minimum, as long as the obstruction is located beyond
the recommended paved shoulder of a roadway, it will have minimum
impact on driver speed or lane position and meet the lateral offset requirement.
Where a curb is present, the lateral offset is measured from the
face of curb and shall be a minimum of 1.5 ft [0.5 m]. A minimum
of 1 ft [0.3 m] lateral offset should be provided from the toe of
barrier to the edge of traveled way.

Clear Zone

A clear recovery area, or clear zone, should be provided along
high-speed rural highways. A clear zone is the unobstructed, traversable
area provided beyond the edge of the through traveled way for the
recovery of errant vehicles. The clear zone includes shoulders,
bike lanes, and auxiliary lanes, except those auxiliary lanes that
function like through lanes. Such a recovery area should be clear of
unyielding objects where practical or shielded by crash cushions
or barrier. Table 2-12 shows criteria for clear zones.

Use above suburban
criteria insofar as available border width permits.

Urban

All (Curbed)

≤ 45

All

4 from curb face

6

Urban

All (Uncurbed)

≥ 50

All

Use above suburban
criteria.

Urban

All (Uncurbed)

≤ 45

All

10

--

1 Because
of the need for specific placement to assist traffic operations,
devices such as traffic signal supports, railroad signal/warning
device supports, and controller cabinets are excluded from clear
zone requirements. However, these devices should be located as
far from the travel lanes as practical. Other non-breakaway devices
should be located outside the prescribed clear zone or these devices
should be protected with barrier.

4 Measured from edge of travel
lane for all cut sections and for all fill sections where side slopes
are 1V:4H or flatter. Where fill slopes are steeper than 1V:4H it
is desirable to provide a 10 ft area free of obstacles beyond the
toe of slope.

5 Desirable, rather than minimum,
values should be used where feasible.

6 Purchase of 5 ft or less of additional
right-of-way strictly for satisfying clear zone provisions is not
required.

NOTE: Online users can view the
metric version of
this table in PDF format.

The clear zone values shown in Table 2-12 are measured from
the edge of travel lane. These are appropriate design values for
all cut sections (see Drainage Facility Placement), for cross sectional design of ditches
within the clear zone area) and for all fill sections with side
slopes 1V:4H or flatter. It should be noted that, while a 1V:4H
slope is acceptable, that a 1V:6H or flatter slope is preferred
for both errant vehicle performance and slope maintainability.
For fill slopes steeper than 1V:4H, errant vehicles have a reduced
chance of recovery and the lateral extent of each roadside encroachment
increases. It is therefore preferable to provide an obstacle-free
area of 10 ft[3.0m] beyond the toe of steep side slopes even when
this area is outside the clear zone.